Question: Two concentric circles are centered at point P. The sides of a 45 degree angle at P form an arc on the smaller circle that is the same length as an arc on the larger circle formed by the sides of a 36  degree angle at P. What is the ratio of the area of the smaller circle to the area of the larger circle? Express your answer as a common fraction.
Answer: Let $C_1$ and $C_2$ be the circumferences of the smaller and larger circle, respectively.  The length of the $45^\circ$ arc on the smaller circle is $\left(\frac{45^\circ}{360^\circ}\right)C_1$, and the length of the $36^\circ$ arc on the larger circle is $\left(\frac{36^\circ}{360^\circ}\right)C_2$.  Setting these two lengths equal we find \[
\frac{C_1}{C_2}=\frac{36}{45}=\frac{4}{5}.
\]The ratio of the areas of the two circles is the square of the ratio of their circumferences: \[
\frac{\pi r_1^2}{\pi r_2^2}=\left(\frac{r_1}{r_2}\right)^2=\left(\frac{2\pi r_1}{2\pi r_2}\right)^2=\left(\frac{C_1}{C_2}\right)^2=\left(\frac{4}{5}\right)^2=\boxed{\frac{16}{25}}.
\]